In such previously developed exposure control circuits, mechanical switches have generally been used in order to initiate the timing interval. Such mechanical switches are operated in response to actuation of the shutter control by the camera operator. While such mechnical switches have provided good service, a need has arisen for an electronic switch which is not subject to mechanical difficulties and which provides a longer, maintenance-free life. Conventional bi-polar electronic devices are not suitable for use as an exposure control switch, since bi-polar devices normally have inherent offset voltages which cause the capacitor to begin its charging cycle at a partially charged state, rather than at zero voltage. Initiation of charging at a partially charged state causes timing errors to be introduced in the timing cycle, since the full charging range of the capacitor is thus not available.
FIG. 1 is a block diagram of a conventional circuit which constitues an electronic switch using bi-polar elements. In FIG. 1, this prior art circuit includes a supply voltage V.sub.cc which is applied to a resistor 10 and one terminal of a timing capacitor 12. The resistor 10 is connected to the collector of a transistor 14, the base of which is connected via lead 16 to one input of a comparator 18. A terminal of the capacitor 12 is connected to the collector of a transistor 20 which has the base thereof connected to the base of transistor 14 and to the lead 16. The emitter of transistor 14 is connected to receive a variable control voltage and the emitter of transistor 20 is connected to receive a reference voltage, in order to allow variation of the timing interval. The collector of transistor 20 is connected via a lead 22 to the second input of the comparator 18. Emitters and collectors of bi-polar transistors 24.sub.a, 24.sub.b, 24.sub.c, . . . 24.sub.n are connected to two terminals of the capacitor 12 and the bases thereof are connected to a Discharge Signal. Transistors 24.sub.a through 24.sub.n are made conductive or non-conductive for automatic exposure control.
In operation, transistor 24 is normally conductive, thereby shoring out capacitor 12 and providing V.sub.cc potential to lead 22 as an input to comparator 18. The resulting output of the comparator 18 maintains the shutter in the closed position when the camera is not being used.
When taking a picture, the shutter is energized and the Discharge Signal makes the transistors 24.sub.a through 24.sub.n non-conductive from conductive immediately before the shutter is opened.
Light is passed through the shutter in order to expose the film. The circuitry illustrated in FIG. 1 controls the exposure time and automatically terminates the exposure. When transistor 24 is made non-conductive, the capacitor 12 begins to charge to a prescribed voltage. When the capacitor charges to a predetermined voltage, the voltage on lead 22 falls to a predetermined level, such that the output of comparator 18 goes low in order to activate a solenoid or the like, not shown, in order to close the shutter and terminate the exposure.
Transistors 14 and 20 are matched and provide a "mirror" current source in order to compensate for variations in the level of V.sub.cc. V.sub.cc variations are cancelled out by the mirrored transistor configuration because the variations are common mode inputs to the comparator. The operation of the illustrated current mirror device is further described in the above-described co-pending patent application, Ser. No. 306,016, filed 9/28/81, now abandoned.
As described above, in the case of constituting an electronic switch using bi-polar elements, a large number of bi-polar elements connected in parallel to a capacitor are required to reduce offset voltage almost to zero. This generates a disadvantage that IC chips require a large area and thus the cost increases.